The specced semantics of GetSubstitution are expected to change in the
case of malformed named references, or named references to nonexistent
named groups. The former will evaluate to the identity replacement of
'$<', while the latter will result in replacement by the empty string.

See also:
https://github.com/tc39/proposal-regexp-named-groups/issues/29

Bug: v8:5437, v8:6912
Cq-Include-Trybots: master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I879288f775774cb0ec563f9d9129a99710efb77c
Reviewed-on: https://chromium-review.googlesource.com/708654
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48426}